Method of fabricating a color cathode ray tube screen



United States Patent 3,440,077 METHOD OF FABRICATING A COLOR CATHODE RAY TUBE SCREEN Joseph J. Dapolito, Lyons, and William D. to Grootenhuis, Seneca Falls, N.Y., assignors to Sylvania Electric Products Inc., a corporation of Delaware No Drawing. Filed Aug. 8, 1966, Ser. No. 570,708 Int. Cl. B44d 1/04, 1/095 US. Cl. 117-18 5 Claims This invention relates to methods of fabricating screens for color television picture tubes and more particularly to methods for depositing uniform layers of phosphor on the tube face as an intermediate step in the fabrication of such screens.

While many methods of applying phosphors to the inside concave face of a cathode ray tube have been proposed, three that have been used with varying degrees of success are: the settling method wherein a liquid cushion or binder is provided over the screen area of the tube and the phosphor is sprayed or otherwise disposed on the upper surface of the cushion and allowed to settle therethrough to the faceplate; the slurry method wherein the phosphor and a binder are mixed together to form a paste which is then spread or flowed upon the faceplate; and the dusting method wherein the screen area of the tube is provided with a coating of a binder material and the phosphor, in a dry powdered form, is dusted over the surface of the binder. This latter form has distinct advantages over the other methods, the main one being that it allows the use of rougher phosphor particles which inherently provide a brighter screen, and it is toward the improvement of this method that this invention is particularly directed.

Generally, when any one of the above-mentioned methods is utilized in the fabrication of a screen for a cathode ray tube suitable for the reconstruction of polychrome images the binder utilized is a material characterized by the fact that its solubility is proportional to a particular form of radiation. One such material, for example, is polyvinyl alcohol sensitized with ammonium or potassium bichromate. This material will alter its solubility upon exposure to actinic radiation and also upon exposure to an electron beam. In this specification, the term radiation shall be understood to include bombardment by electron beam as well as by rays existing in the optical spectrum.

To produce a color cathode ray tube of the type used currently it is necessary to apply to the faceplate of the tube a plurality of sets of discrete elements of phosphors capable of emitting different colors of light upon excitation by an electron beam. The most common type now in use uses three different phosphors capable of emitting red, green, and blue light. These three colors are arranged in triads over the face of tube, and, in one current tube type their numbers may exceed one million.

These sets of discrete elements are adhered by applying to the screen area an admixture of phosphor and radiation sensitive binder by any one of the above-mentioned methods. The screen so coated is mated with a suitable negative and exposed to radiation from a point source to form a pattern on the screen. After exposure the screen is developed in a manner well known in the art to leave a plurality of discrete elements on the screen. This procedure is repeated a total of three times (for a three color screen) with the point source of radiation in a different position each time and then the screen is assembled to a funnel shaped body portion and further processing takes place, all as is well known in the art.

In the prior methods of dusting screens occasional problems arose which deleteriously affected the finished tube. These problems manifested themselves as pick- "ice outs, i.e., areas where the phosphor did not adhere well to the screen and subsequently fell off leaving bare spots, and unequal light output caused by uneven distribution of the phosphor which resulted in varying thicknesses of the discrete elements.

It is, therefore, an object of this invention to enhance the fabrication techniques of color cathode ray tube screens.

It is another object of the invention to obviate the above-cited disadvantages.

It is another object of the invention to reduce the cost of manufacturing color cathode ray tubes by reducing the number of faulty screens.

Still another object of the invention is the provision of a color cathode ray tube having substantially equal brightness over the entire face thereof.

These and other objects are accomplished, in one aspect of the invention, by the provision of a method for fabricating, upon a support, a screen for a cathode ray tube suitable for the reconstruction of polychrome images which screen comprises sets of discrete elements of at least two different colored light emitting phosphors affixed thereon by a radiation exposure technique. The improvement resides in the manner of applying to the support an admixture of phosphor particles and a material whose solubility is proportional to particular radiation prior to the formation of the discrete elements. The method comprises cleaning the support and then applying thereto a tacky material sensitive to a particular form of radiation exposure. After application of the material, and while it is still tacky, the support is positioned at a phosphor receiving station. At least the area immediately adjacent the phosphor receiving station is provided with a controlled atmosphere having particular pressure, temperature, and humidity requirements. The support is then surrounded by a phosphor cloud confining tub and dry phosphor particles and substantially contaminant free air are drawn from respective supplies thereof, mixed in a mixing manifold to provide a suspension of the particles and dispensed from a dispensing gun to form a cloud within the tub. A prescribed amount of phosphor particles is deposited on the tacky material, the support being rotated about an axis of rotation during this deposition whereby a substantially uniform distribution of the phosphor particles is achieved. After the prescribed amount of phosphor has been deposited the rotation is stopped, the tub removed, and the remainder of the phosphor cloud is siphoned away for subsequent use.

This method produces superior screens for color cathode ray tubes that are characterized by substantial freedom from pick-outs and substantially equal brightness over the entire screen. Also, since the amount of screen rejections is reduced, the cost of the finished tube is likewise favorably affected.

For a better understanding of the present invention, together with other and further objects, advantages, and capabilities thereof, reference is made to the following disclosure and appended claims.

Referring now to the method with greater particularity, the support upon which the screen is to be laid, and which preferably is the inner or concave surface of a cathode ray tube faceplate, is first thoroughly cleaned. The cleaning may be accomplished by successive rinses of alcohol followed by rinses of distilled or deionized water. Rinses with a mildly akaline solution followed by a rinse with a weak acidic solution are also effective; however, thorough rinsing with distilled or deionized water must follow any of the above procedures. If an acid bath containing any halogen ions is utilized the rinsing must be suflicient to insure removal thereof since these extremely reactive ions will poison the subsequently applied phosphor.

After the support is cleaned and dried it is necessary that the following operations take place in a controlled environment where any form of contamination is held to a minimum. Preferably the cleaning and drying also take place in this environment.

To begin with, the air pressure within the environment, which may be a specially constructed area or room, is maintained at some level greater than normal atmospheric pressure. This pressure difference need not be excessive and, since normal atmospheric pressure at sea level is roughly 14.7 lbs. per sq. in., a pressure of 15 lbs. per sq. in. is sufficient. This slight pressure differential insures that no contaminated air from the exterior of the environment will flow into it.

The cleaned and dried support is then placed in a second environment for coating with the tacky, radiation explosure sensitive material. The material is preferably applied by spraying upwardly to the support and the coating is built up gradually by successive passes of spray rather than singly. The multiple spraying passes allows a more uniform coating to be applied and allows better control of the tackiness. To further insure against the coating drying out the second environment contains a saturated atmosphere which keeps the coating at the required level of tackiness for at least a prescribed period of time, say approximately three minutes. The subsequent dusting of the phosphor must be accomplished within this time period.

Within the general environment it is necessary to keep the relative humidity levels and temperature within specific ranges. A relative humidity between 40% and 50% and a temperature range between 70 F. and 80 F. has been found to be effective in preventing some of the more common problems inherent in screen production. The humidity range helps to keep the radiation exposure sensitive material from drying out at too rapid a rate and this temperature range has been found to be most ideal for substantially eliminating some forms of phosphor contamination which have been traced to excessive perspiration on the part of screening employees. It has been discovered that the chloride ion present in perspiration can sufficiently poison phosphors so as to result in rejected screens. So insidious and devastating is chloride poisoning to the phosphors currently being used that it has been found necessary to eliminate from the screening areas trays and containers made from the plastic material polyvinyl chloride.

To eliminate the formation of pick-outs it is necessary to understand the causes. One of the prime causes is airborne contaminants that fall onto the tacky material and absorb some of the solvent therein thus causing dry spots. When the phosphor particles are subsequently applied adherence at these areas will not be sufficient to maintain good contact with the support and thus the discrete elements formed at those areas will become loosened and fall away leaving bare spots in the screen. Among the most common of these air-borne contaminants is lint. To

eliminate this from the screening environment it is necessary that anyone working within or entering this environment be provided with lint free clothing, Dacron gowns being found satisfactory for this purpose.

After being provided with the tacky coating of the radiation exposure sensitive material the support is positioned at a phosphor receiving station with the tacky coating uppermost. A phosphor cloud confining tub which has an opening in the upper wall thereof is placed around the support to form a surrounding enclosure.

Dry phosphor particles are drawn from a supply thereof to a mixing manifold wherein they are mixed with substantially contaminant free air that has been filtered to eliminate any particles of a size larger than seven microns and preferably it should be free of particles of a size larger than three microns. The air should also be of very low relative humidity and this may be accomplished by passing it between heaters during its route from its supply to the mixing manifold. Preferably, the relative humidity should be less than 5%. The temperature of the air should not be excessive and preferably it should be between 70 F. and F.

A phosphor dispensing gun is now introduced to the interior of the phosphor cloud confining tub through the opening in the upper wall thereof and rotation of the support and tub is started. The speed of rotation is dependent upon many factors including the particle size of the phosphor, the thickness of the layer desired and the pufiing cycle; however, the speed should not be greater than about six revolutions per minute to avoid the creation of air turbulence and excessive centrifugal forces which might cause the phosphor to build up at the edges of the support. It is desirable that no less than one complete revolution of the panel be utilized to insure even distribution of the phosphor particles. The suspension of phosphor particles and air which was formed in the mixing manifold is now dispensed from the gun in bursts or puffs. These puffs are directed in a plane substantially normal to the longitudinal axis of the support so as to avoid any drying out of the tacky material which might occur if the puffs of air and phosphor were directed downwardly.

The putting and rotation continue until a sufficient amount of phosphor is deposited on the tacky coating after which these operations cease. The cut-off of these operations may be automatically controlled, e.g. by the use of an optical pair, i.e., a photoconduetive cell and a light source. By positioning the light source beneath the support and the photoconduetive cell above the support, say at the dispensing gun, and aligned with the light source it is possible to use the photocell to signal cut-off when the light level falling thereon decreases to a given level caused by the phosphor cloud and the phosphor depositing on the support.

After rotation of the support and the dispensing of the phosphor have stopped the phosphor cloud confining tub is removed and any undeposited phosphor remaining in the deposition area is siphoned off to a recovery area where it may be reprocessed for subsequent use.

The phosphor coated support is now removed from the phosphor receiving station and mated with a negative, which may be the foraminated shadow mask of the tube, and it is positioned at an exposure station. The radiation exposure source is a point source and it is positioned in an offset manner in approximately the location, relative to the support, that the electron gun associated with this particular phosphor will occupy in the finished tube. Upon energization the radiation leaves the source, passes through the holes in the mask and impinges upon the screen. The solubility of the radiation exposure sensitive material at these areas becomes altered in a particular manner dependent upon the material. If the material is sensitized polyvinyl alcohol the impinged areas are polymerized and they become less soluble, in a particular solvent, than the unexposed areas. After exposure the support is removed from the exposure station and developed which, in this instance, comprises washing the support with deionized waterthe solvent for sensitized polyvinyl alcohol-40 remove the unexposed areas of material. After the developing, discrete elements of phosphor and hardened polyvinyl alcohol remain adhered to the support.

The support is then dried and the entire sequence of operations, beginning with the spraying on of the radiation exposure sensitive material, is repeated, this time using a different phosphor. At the exposure table the radiation point source is offset to another position corresponding approximately to the position the electron gun associated with this phosphor will assume in the finished tube.

The above operations are repeated as many times as necessary, usually three for the common color tube in use today, and then the support is assembled to the tube.

and the polymerized polyvinyl alcohol is volatilized and exhausted from the tube.

It will be seen that this method of fabrication produces many advantages over the prior art. The control of the pressures and temperatures and humidity greatly reduces the causes of pick-outs and thus provides a much better screen. The pufiing technique together with the rotation of the support during the deposition cycle and the substantially evenly tacky coating provides an extremely uniform phosphor layer which is reflected in the production of tubes having substantially even light distribution over the entire face thereof.

Further, the so prepared phosphor coated support is completely adaptable to either a photographic fixing technique wherein the exposure is performed by actinic radiation or an electron fixing technique wherein the exposure is performed by an electron beam. Obviously, in the latter technique the exposure must take place in a demountable tube.

Also, while only one type of radiation exposure sensitive material has been described; namely, polyvinyl alcohol, it is to be noted that many other materials are known to the art which will perform substantially as well.

What is claimed is:

1. In a method of fabricating upon the concave surface of the faceplate of a cathode ray tube a screen suitable for the reconstruction of polychrome images which screen comprises sets of discrete elements of at least two different colored light emitting phosphors affixed thereon by a photographic technique, the improvement which comprises the method of providing an admixture of phosphor particles and a photosensitized material on said concave surface prior to the formation of said discrete patterns which comprises the steps of: cleaning said concave surface of said faceplate; applying to said concave surface a tacky, photosensitized material in a manner to provide a substantially uniform thickness thereof and under conditions whereby said photosensitized material will remain, for at least a prescribed time, in a condition to provide adherence of phosphor particles thereto; positioning said concave surface at a phosphor receiving station; providing at least the area immediately adjacent said phosphor receiving station with a controlled atmosphere having a temperature in the range of about 70 to 80 F. and a relative humidity in the range of about 40% to 50%; surrounding said concave surface with a phosphor cloud confining tub having an opening in a wall thereof diametrically opposed to and remote from said concave surface; drawing dry phosphor particles from a supply thereof to a mixing manifold; drawing substantially contaminant free air containing no impurities of a size larger than 7 microns of a given temperature and humidity from a supply thereof to said mixing manifold; mixing said phosphor particles and said air to provide a suspension of particles; introducing a phosphor dispensing gun through said opening to a position within said tub; feeding said suspension of phosphor particles from said manifold to said gun; dispensing said suspension from said gun to form a cloud within said tub; depositing on the photosensitized material a prescribed amount of phosphor particles; rotating said concave surface and said tub at a given speed about an axis of rotation during the dispensing of said suspension whereby substantially uniform distribution of said phosphor particles is achieved; and removing said tub and siphoning off the remainder of said phosphor cloud for subsequent use after said prescribed amount of phosphor particles has been deposited.

2. The method of claim 1 wherein said photosensitized material is applied by spraying in a saturated atmosphere.

3. The method of claim 1 wherein said speed of rotation is less than six revolutions per minute.

4. In a method of fabricating, upon a support, a screen for a cathode ray tube suitable for the reconstruction of polychrome images which screen comprises sets of discrete elements of at least two different colored light emitting phosphors afiixed thereon by a radiation exposure technique, the improvement which comprises the method of providing an admixture of phosphor particles and material whose solubility is proportional to particular radiation on said support prior to the formation of said discrete elements which comprises the steps of: cleaning said support; applying to said support a tacky material sensitive to a particular form of radiation exposure in a manner to provide a substantially uniform thickness thereof and under conditions whereby said material will remain, for at least a prescribed time, in a condition to provide adherence of phosphor particles thereto; positioning said support at a phosphor receiving station; providing at least the area immediately adjacent said phosphor receiving station with a controlled atmosphere having a temperature in the range of about to F. and a relative humidity in the range of about 40% to 50%; surrounding said support with a phosphor cloud confining tub having an opening in a wall thereof; drawing dry phosphor particles from a supply thereof to a mixing manifold; drawing substantially contaminant free air from a supply thereof to said mixing manifold; mixing said phosphor particles and said air to provide a suspension of particles; introducing a phosphor dispensing gun through said opening to a position within said tub; feeding said suspension of phosphor particles from said manifold to said gun; dispensing said suspension from said gun to form a cloud within said tub; depositing on the material a prescribed amount of phosphor particles; rotating said support and said tub at a given speed about an axis of rotation during the dispensing of said suspension whereby substantially uniform distribution of said phosphor particles is achieved; and removing said tub and siphoning off the remainder of said phosphor cloud for subsequent use after said prescribed amount of phosphor particles has been deposited.

5. The method of claim 4 wherein said solubility of said material is sensitive to exposure to an electron beam.

References Cited UNITED STATES PATENTS 2,690,979 10/1954 Law 11725 XR 2,844,758 7/1958 Dow 1l733.5 XR 2,854,348 9/1958 Bowerman 11733.5 XR 2,903,377 9/1959 Saulnier l17-33.5 XR 2,959,483 11/1960 Kaplan 117-335 XR 3,025,161 3/1962 Rychlewski l1733.5 XR 3,080,231 3/1963 Perry et al.

ALFRED L. LEAVITT, Primary Examiner.

WAYNE F. CYRON, Assistant Examiner.

US. Cl. X.R. 117-333, 33.5 

1. IN A METHOD OF FABRICATING UPON THE CONCAVE SURFACE OF THE FACEPLATE OF A CATHODE RAY TUBE A SCREEN SUITABLE FOR THE RECONSTRUCTION OF POLYCHROME IMAGES WHICH SCREEN COMPRISES SETS OF DISCRETE ELEMENTS OF AT LEAST TWO DIFFERENT COLORED LIGHT EMITTING PHOSPHORS AFFIXED THEREON BY A PHOTOGRAPHIC TECHNIQUE, THE IMPROVEMENT WHICH COMPRISES THE METHOD OF PROVIDING AN ADMIXTURE OF PHOSPHOR PARTICLES AND A PHOTOSENITIZED MATERIAL ON SAID CONCAVE SURFACE PRIOR TO THE FORMATION OF SAID DISCRETE PATTERNS WHICH COMPRISES THE STEPS OF: CLEANING SAID CONCAVE SURFACE OF SAID FACEPLATE; APPLYING TO SAID CONCAVE SURFACE A TACKY, PHOTOSENSITIZED MATERIAL IN A MANNER TO PROVIDE A SUBSTANTIALLY UNIFORM THICKNESS THEREOF AND UNDER CONDITIONS WHEREBY SAID PHOTOSENSITIZED MATERIAL WILL REMAIN, FOR AT LEAST A PRESCRIBED TIME, IN A CONDITION TO PROVIDE ADHERENCE OF PHOSPHOR PARTICLES THERETO; POSITIONING SAID CONCAVE SURFACE AT A PHOSPHOR RECEIVING STATION; PROVIDING AT LEAST THE AREA IMMEDIATELY ADJACENT SAID PHOSPHOR RECEIVING STATION WITH A CONTROLLED ATMOSPHERE HAVING A TEMPERATURE IN THE RANGE OF ABOUT 70* TO 80*F. AND A RELATIVE HUMIDITY IN THE RANGE OF ABOUT 40% TO 50%; SURROUNDING SAID CONCAVE SURFACE WITH A PHOSPHOR CLOUD CONFINING TUB HAVING AN OPENING IN A WALL THEREOF DIAMETRICALLY OPPOSED TO AND REMOTE FROM SAID CONCAVE SURFACE; DRAWING DRY PHOSPHOR PARTICLES FROM A SUPPLY THEREOF TO A MIXING MANIFOLD; DRAWING SUBSTANTIALLY CONTAMINANT FREE AIR CONTAINING NO IMPURITIES OF A SIZE LARGER THAN 7 MICRONS OF A GIVEN TEMPERATURE AND HUMIDITY FROM A SUPPLY THEREOF TO SAID MIXING MANIFOLD; MIXING SAID PHOSPHOR PARTICLES AND SAID AIR TO PROVIDE A SUSPENION OF PARTICLES; INTRODUCING A PHOSPHOR DISPENSING GUN THROUGH SAID OPENING TO A POSITION WITHIN SAID TUB; FEEDING SAID SUSPENSION OF PHOSPHOR PARTICLES FROM SAID MANIFOLD TO SAID GUN; DISPENSING SAID SUSPENSION FROM SAID GUN TO FORM A CLOUD WITHIN SAID TUB; DEPOSITING ON THE PHOTOSENSITIZED MATERIAL A PRESCRIBED AMOUNT OF PHOSPHOR PARTICLES; ROTATING SAID CONCAVE SURFACE AND SAID TUB AT A GIVEN SPEED ABOUT AN AXIS OF ROTATION DURING THE DISPENSING OF SAID SUSPENSION WHEREBY SUBSTANTIALLY UNIFORM DISTRIBUTION OF SAID PHOSPHOR PARTICLES IS ACTIVED; AND REMOVING SAID TUB AND SIPHONING OFF THE REMAINDER OF SAID PHOSPHOR CLOUD FOR SUBSEQUENT USE AFTER SAID PRESCRIBED AMOUNT OF PHOSPHOR PARTICLES HAS BEEN DEPOSITED. 